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Seasonal Ice-Speed Variations in 10 Marine- Terminating Outlet Glaciers Along the Coast of Prudhoe Land, Northwestern Greenland

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Seasonal Ice-Speed Variations in 10 Marine- Terminating Outlet Glaciers Along the Coast of Prudhoe Land, Northwestern Greenland Journal of Glaciology Seasonal ice-speed variations in 10 marine- terminating outlet glaciers along the coast of Prudhoe Land, northwestern Greenland Paper Daiki Sakakibara1,2 and Shin Sugiyama2,1 Cite this article: Sakakibara D, Sugiyama S 1Arctic Research Center, Hokkaido University, Kita-21, Nishi-11, Kita-ku, Sapporo, 001-0021, Japan and 2Institute of (2020). Seasonal ice-speed variations in 10 Low Temperature Science, Hokkaido University, Kita-19, Nishi-8, Kita-ku, Sapporo, 060-0819, Japan marine-terminating outlet glaciers along the coast of Prudhoe Land, northwestern Greenland. Journal of Glaciology 66(255), Abstract 25–34. https://doi.org/10.1017/jog.2019.81 We present a 3-year record of seasonal variations in ice speed and frontal ablation of 10 marine- Received: 18 April 2019 terminating outlet glaciers along the coast of Prudhoe Land in northwestern Greenland. The Revised: 14 October 2019 glaciers showed seasonal speedup initiated between late May and early June, and terminated Accepted: 14 October 2019 between late June and early July. Ice speed subsequently decreased from July to September. First published online: 13 November 2019 The timing of the speedup coincided with the onset of the air temperature rise to above freezing, Key words: suggesting an influence of meltwater availability on the glacier dynamics. No clear relationship Arctic glaciology; glacier monitoring; ice was found between the speedup and the terminus position or the sea-ice/ice-mélange conditions. velocity; remote sensing These results suggest that the meltwater input to the glacier bed triggered the summer speedup. The excess of summer speed (June–August) over the mean for the rest of the year accounted for Author for correspondence: – Daiki Sakakibara, E-mail: sakakibara@pop. 0.5 13% of the annual ice motion. Several glaciers showed seasonal frontal variations, i.e. retreat lowtem.hokudai.ac.jp in summer and advance in winter. This was not due to ice-speed variations, but was driven by seasonal variations in frontal ablation. The results demonstrate the dominant effect of glacier surface melting on the seasonal speedup, and the importance of seasonal speed patterns on longer-term ice motion of marine-terminating outlet glaciers in Greenland. 1. Introduction Mass loss from the Greenland ice sheet has contributed to global sea level rise at a mean rate of − 0.47 mm a 1 during the period from 1991 to 2015 (van den Broeke and others, 2016). Since ∼40% of the mass loss was due to changes in ice discharge from marine-terminating outlet glaciers (van den Broeke and others, 2016), a future projection of ice discharge from the Greenland ice sheet is crucial for understanding the magnitude and timing of global sea level rise. Ice discharge from the ice sheet is dependent on the ice speed of marine-terminating outlet glaciers. Glaciers in Greenland had accelerated substantially in the early 2000s, and ice speed has been kept at the accelerated level since 2005 (e.g. Howat and others, 2008; Moon and others, 2012; King and others, 2018; Mankoff and others, 2019). Observed speed changes are inhomogeneous in space and time; thus further investigations and insights are needed to pro- ject future changes in marine-terminating outlet glaciers in view of our changing climate. Recent progress in satellite remote sensing techniques contributes to the studies of seasonal speed patterns of marine-terminating glaciers in Greenland (Howat and others, 2010; Moon and others, 2014, 2015; Vijay and others, 2019). Reported data indicate significant summer acceleration, suggesting the importance of high-frequency ice-speed measurements to the quantification of annual ice discharge. Sole and others (2011) reported the ∼1% contribution of summer speedup to annual ice motion of a marine-terminating outlet glacier in southwest Greenland. However, as the study site was far inland from the glacier terminus at >1200 m a.s.l., the importance of summer speedup to glacier dynamics in the vicinity of the terminus remains poorly understood. Seasonal speed variations are essential also for understanding the response of ice dynamics to external forcings (Howat and others, 2010; Carr and others, 2013; Moon and others, 2014; Lemos and others, 2018; Vijay and others, 2019). Moon and others (2014) reported the sea- sonal speed patterns of 55 marine-terminating glaciers along the coast of northwest, west, southeast and southwest Greenland. This study classifies seasonal speed patterns into three types, depending on their sensitivity to the terminus position, meltwater production and the subglacial hydrological system. Seasonal speed patterns in northwestern Greenland were correlated with meltwater production, suggesting a higher sensitivity of the glacier dynamics in this region to climate variability (Moon and others, 2014). More recently, higher frequency © The Author(s) 2019. This is an Open Access ice-speed measurements were performed on 45 glaciers over broader areas in Greenland (Vijay article, distributed under the terms of the Creative Commons Attribution licence (http:// and others, 2019). This study shows the concurrence of summer speedup and the onset of gla- creativecommons.org/licenses/by/4.0/), which cier surface melting, which essentially agrees with the conclusion by Moon and others (2014). permits unrestricted re-use, distribution, and These studies indicate the importance of expanding the spatial and temporal coverage of these reproduction in any medium, provided the data to include the investigation of future responses of Greenlandic outlet glaciers to changing original work is properly cited. climate. Seasonal speed variations in marine-terminating glaciers are influenced by several factors such as subglacial hydraulic conditions, surface melting and drainage into the glacier bed, ter- cambridge.org/jog minus variations and sea-ice/ice-mélange conditions (e.g. Howat and others, 2010; Carr and Downloaded from https://www.cambridge.org/core. 30 Sep 2021 at 14:27:26, subject to the Cambridge Core terms of use. 26 Daiki Sakakibara and Shin Sugiyama others, 2013; Moon and others, 2014; Pimentel and others, 2017). ∼20 km wide fjord Inglefield Bredning. Hubbard, Bowdoin, Numerical modeling and observational studies indicate the Verhoeff, Morris Jesup and Diebitsch Glaciers are located in the importance of the glacier terminus position in ice dynamics western half of the study area, where each individual fjord con- (Howat and others, 2008; Joughin and others, 2008a, 2008b; nects to western part of Inglefield Bredning or directly to northern Nick and others, 2009). Observations in southeast Greenland Baffin Bay. The ice speed near the termini of these glaciers ranges − have shown that resistive stress acting on the terminus is dimin- from 90 to 400 m a 1 (Sakakibara and Sugiyama, 2018), which is ished when a marine-terminating glacier retreats, a process smaller than the speed of the glaciers in the eastern half. Heilprin, which is significant enough to cause flow acceleration (Howat Farquhar, Bowdoin and Diebitsch Glaciers showed a substantial and others, 2008). Numerical experiments on Helheim Glacier speedup and retreated rapidly after 2000 (Sakakibara and confirmed the critical effect on glacier dynamics of bed geometry Sugiyama, 2018). near the terminus (Nick and others, 2009). Sea ice and ice mélange in front of a marine-terminating glacier are thought to have a buttressing effect on the glacier front (Joughin and others, 3. Data and methods 2008a; Amundson and others, 2010). Therefore, the melting of To measure the glacier terminus position, ice speed and sea-ice/ sea ice and ice mélange has the potential to reduce resistive stress ice-mélange conditions from 2014 to 2016, we used satellite acting on the glacier and cause summer speedup. The meltwater images of the Landsat 8 Operational Land Imager (OLI) (level input to the glacier bed and the evolution of a subglacial drainage 1T, with 15/30 m resolutions) distributed by the United States system also influence seasonal speed variations (Joughin and Geological Survey. Because the OLI is an optical sensor, the mea- others, 2008b; Schoof, 2010; Sole and others, 2011). In early sum- surements were performed when sunlight was available between mer, surface melt increases and water drains to the bed, where a March and October. subglacial drainage system is not sufficiently developed. In such a We obtained hourly air temperatures at Mittarfik Qaanaaq situation, subglacial water pressure increases and basal sliding is (Qaanaaq Airport; 77.48°N, 69.38°W; 16 m a.s.l.) located within enhanced. The drainage system develops further as a greater 30–84 km of the glacier termini, from the National Oceanic and amount of meltwater drains, resulting in a drop in subglacial Atmospheric Administration. The temperature dataset was uti- water pressure and a decrease in ice speed (e.g. Bartholomew lized to calculate mean air temperature during a period for each and others, 2010; Sundal and others, 2011). These mechanisms ice-speed measurement. The Qaanaaq temperature showed a have been proposed and tested at some of the glaciers in good correlation (r2 = 0.94, p < 0.001) with that recorded at Greenland, but processes affecting seasonal speed variations are Thule Airbase located 108 km south from Qaanaaq. Thus, we not yet fully understood. assume the data well represent air temperature over the study area. To investigate seasonal speed variations of marine-terminating outlet glaciers, we processed satellite images for high-frequency ice-speed variations in 10 marine-terminating outlet glaciers 3.1. Ice speed along the coast of the Prudhoe Land, northwestern Greenland. We compared observed ice-speed changes in years 2014, 2015 The ice speed was measured by automatically tracking glacier sur- and 2016 with the terminus position, sea-ice/ice-mélange condi- face features on temporally separated satellite-image pairs tions and air temperature in order to discover the source of sea- (Scambos and others, 1992; Sakakibara and Sugiyama, 2014, sonal speed variations. The contribution of summer speedup to 2018). We used Landsat 8 OLI images obtained from 2014 to annual ice motion was quantified to evaluate its importance to 2016 for this purpose.
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